Implantable medical devices are available for monitoring physiological signals for use in diagnosing and managing cardiac disease. For example, implantable hemodynamic monitors can monitor heart rhythm, blood pressure and thoracic fluid status for tracking the status of heart failure patients. In the early stages of heart failure, compensatory mechanisms occur in response to the heart's inability to pump a sufficient amount of blood. One compensatory response is an increase in filling pressure of the heart. The increased filling pressure increases the volume of blood in the heart, allowing the heart to more efficiently eject a larger volume of blood on each heart beat. Increased filling pressure and other compensatory mechanisms can initially occur without overt heart failure symptoms.
The mechanisms that initially compensated for insufficient cardiac output lead to heart failure decompensation as the heart continues to weaken. The weakened heart can no longer pump effectively causing increased filling pressure to lead to chest congestion (thoracic edema) and heart dilation, which further compromises the heart's pumping function, and the patient begins the “vicious cycle” of heart failure which generally leads to hospitalization. By detecting or predicting heart failure decompensation early, even before the patient becomes overtly symptomatic, hospitalization may be avoided through careful therapy management.